1. Field of the Invention
The present invention relates to a method of producing a diamond. More particularly, the present invention relates to a method of producing a colorless, single-crystal diamond at a rapid growth rate using Microwave Plasma Chemical Vapor Deposition (MPCVD) within a deposition chamber.
2. Description of Related Art
Large-scale production of synthetic diamond has long been an objective of both research and industry. Diamond, in addition to its gem properties, is the hardest known material, has the highest known thermal conductivity, and is transparent to a wide variety of electromagnetic radiation. Therefore, it is valuable because of its wide range of applications in a number of industries, in addition to its value as a gemstone.
For at least the last twenty years, a process of producing small quantities of diamond by chemical vapor deposition (CVD) has been available. As reported by B. V. Spitsyn et al. in “Vapor Growth of Diamond on Diamond and Other Surfaces,” Journal of Crystal Growth, vol. 52, pp. 219-226, the process involves CVD of diamond on a substrate by using a combination of methane, or another simple hydrocarbon gas, and hydrogen gas at reduced pressures and temperatures of 800-1200° C. The inclusion of hydrogen gas prevents the formation of graphite as the diamond nucleates and grows. Growth rates of up to 1 μm/hour have been reported with this technique.
Subsequent work, for example, that of Kamo et al. as reported in “Diamond Synthesis from Gas Phase in Microwave Plasma,” Journal of Crystal Growth, vol. 62, pp. 642-644, demonstrated the use of Microwave Plasma Chemical Vapor Deposition (MPCVD) to produce diamond at pressures of 1-8 kPa at temperatures of 800-1000° C. with microwave power of 300-700 W at a frequency of 2.45 GHz. A concentration of 1-3% methane gas was used in the process of Kamo et al. Maximum growth rates of 3 μm/hour have been reported using this MPCVD process. In the above-described processes, and in a number of other reported processes, the growth rates are limited to only a few micrometers per hour.
Until recently, known higher-growth rate processes have only produced polycrystalline forms of diamond. However, new methods of improving the growth rates of single-crystal chemical vapor deposition (SC-CVD) diamonds have recently been reported, and these methods have opened new opportunities for the application of diamond for gems, optics, and electronics [1,2]. Several other groups have started to grow SC-CVD diamonds [3, 4, 5]. SC-CVD diamonds reported so far, however, are relatively small, are discolored, and/or are flawed. Large (e.g., over three carats, as commercially available high pressure, high temperature (HPHT) synthetic Ib yellow diamond), colorless, flawless synthetic diamonds remain a challenge due to slow growth and other technical difficulties [7, 8, 9]. The color of SC-CVD diamonds in the absence of HPHT annealing can range from light brown to dark brown, thus limiting their applicability as gems, in optics, in scientific research, and in diamond-based electronics [6, 7, 8]. SC-CVD diamonds have been characterized as type IIa, i.e., possessing less than 10 ppm nitrogen, and have coloration and other optical properties arising from various defects and/or impurities.
Single-crystal brown SC-CVD diamonds with 4.5 mm in thickness can be produced at high growth rates of about 100 micrometers/hour with nitrogen added, and deposited on cut-off SC-CVD seed instead of natural or HPHT synthetic substrates [1,2]. A diamond crystal of 10 carats is approximately five times that of commercially available HPHT diamond and the SC-CVD diamond reported in References [7, 8, 9, 10]. Single-crystal diamonds with larger mass (greater than 100 carats) are needed as anvils for high-pressure research, and crystals with large lateral dimensions (greater than 2.5 cm) are required for applications such as laser windows and substrates for diamond-based electronic devices. High optical quality (UV-visible-IR transmission) and chemical purity are required for all of the above applications. The large SC-CVD diamonds produced so far present problems because of the brownish color.
Attempts have been made to add oxygen in the growth of polycrystalline CVD diamond. These effects include extending the region of diamond formation [12], reducing silicon and hydrogen impurity levels [13], preferentially etching the non-diamond carbon [11, 14], and attempting to prevent diamond cracks due to an absence of impurities [13]. These attempts were directed to etching and the synthesis of polycrystalline diamonds but not to the production of SC-CVD diamond.
U.S. Pat. No. 6,858,078 to Hemley et al. is directed to an apparatus and method for diamond production. The disclosed apparatus and method, although pioneering as a means of rapidly producing single-crystal CVD diamonds, can lead to the production of diamonds with a light brown color.
Thus, there remains a need to produce large, high quality, single-crystal diamonds at a rapid growth rate and to produce them colorless (i.e., high UV-visible-IR transmission).